The broad goal of Project 2 is to establish roles played by nicotinic acetylcholine receptors (nAChR) in nicotine dependence and its treatment. As part of the integrated program of research proposed as an NCDDG-MD/NA, the more narrow goal of the project is to define profiles for functional interaction with diverse nAChR subtypes that are critical to drug efficacy in treatment of nicotine dependence. The focus on nAChR subtypes, defined by their different subunit compositions, is justified because they play critical, broad, and diverse roles in synaptic and nervous system function and are likley to be involved in nicotine dependence. Bupropion is a model substance for these studies, having proven clinical efficacy as a smoking cessation aid, but also having activity as a functional nAChR antagonist. The research program's central hypothesis is that agents effective in treatment of nicotine dependence will share with bupropion (and perhaps its metabolites) the ability to act with comparable potencies at multiple molecular targets. It is postulated that drug interactions at selected nAChR subtypes and with dopamine and/or norepinephrine transporters integrate to produce a neurochemical outcome that is an effective substitute for nervous system actions of chronic nicotine or tobacco product use. Project 2 will specifically test the hypothesis that potent drug action as an antagonist of the cx4132-nAChR subtype is a feature of drugs with high smoking cessation potential. To test this hypothesis, and to help elucidate sites and mechanisms of action involved, studies will be done to define interactions of bupropion, compounds of the 3-phenyltropane series, their analogs, and their possible metabolites with diverse nAChR subtypes naturally or heterologously expressed by human clonal cell line models. Specific Aim 1 is to efficiently establish acute functional antagonist potencies of drugs at o_4I_2-nAChR through a high throughput screening process employing isotopic ion flux assays complemented and validated by results from whole cell current recording. Specific Aim 2 is to provide a more detailed nAChR interaction profile for drugs meeting selection criteria from Aim 1 and from the other projects in the research program. These studies will establish, for selected agents, acute effects on 0_7-, c_3"- and other nAChR subtypes, effects of longer-term drug treatment on nAChR function, and whether selected agents act as competitive or non-competitive antagonists. These studies will be repeated to define nAChR subtype activity for any new chemical entities developed in the research program. These studies are significant because they will delineate contributions, if any, of nAChR function in actions of bupropion and other ligands with smoking cessation utility or potential. Definition of nAChR subtype interaction profiles correlating with or predictive of drug utility in treatment of nicotine dependence also is expected and is anticipated to have siqnificant druq discovery value. Mechanisms involved in nicotine dependence also will be clarified.